The present invention relates generally to valve train mechanisms for internal combustion engines and, more particularly, to a variable lift valve train mechanism.
Valve "lift" may be defined as the degree of opening movement of an intake or exhaust poppet valve with movement of a valve actuator such as a cam lobe having a given profile. In this context, the term "lift" does not imply movement in a particular direction (up or down). Typical static (non-variable) lift valve trains for internal combustion engines are designed as a compromise with reference to engine performance at various RPMs. Generally, the more powerful engines are designed with a relatively higher valve lift camshaft and develop their maximum horsepower and torque at higher RPMs, with a sacrifice in terms of overall fuel economy. Conversely, the more fuel efficient engines generally have relatively lower valve lift camshafts and concentrate their horsepower and torque at lower engine RPMs, but lack the ability to generate high usable power. In short, conventional engine design has the effect of positioning a point of maximum volumetric efficiency either higher up or lower down on the RPM scale.
A number of variable lift valve train mechanisms have been proposed, typical examples being disclosed in Hara et al U.S. Pat. No. 4,526,142, Hara et al U.S. Pat. No. 4,498,432, and Bonvallet U.S. Pat. No. 4,638,773. Generally, in such mechanisms, a rocker arm is positioned so that one end thereof is actuated either directly by a cam, as in an overhead cam engine, or by a cam-actuated push rod. The other end of the rocker arm engages the free stem end of a poppet valve, such as an intake valve or an exhaust valve. The upper surface of the rocker arm has a contoured portion which abuts an upper, plate-like reaction member, with the contact point between the rocker arm and the reaction member serving as the pivot or fulcrum point of the rocker arm. Various arrangements have been proposed for varying the angle or orientation of the upper reaction member, thereby to alter the pivot or fulcrum point and thus vary valve lift.